A Protobuf Msg service processes messages. Protobuf Msg services are specific to the module in which they are defined, and only process messages defined within the said module. They are called from BaseApp during DeliverTx.

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Pre-requisite Readings​

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Implementation of a module Msg service​

Each module should define a Protobuf Msg service, which will be responsible for processing requests (implementing sdk.Msg) and returning responses. As further described in ADR 031, this approach has the advantage of clearly specifying return types and generating server and client code. Protobuf generates a MsgServer interface based on a definition of Msg service. It is the role of the module developer to implement this interface, by implementing the state transition logic that should happen upon receival of each sdk.Msg. As an example, here is the generated MsgServer interface for x/bank, which exposes two sdk.Msgs: x/bank/types/tx.pb.go 
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See full example on GitHub When possible, the existing module’s Keeper should implement MsgServer, otherwise a msgServer struct that embeds the Keeper can be created, typically in ./keeper/msg_server.go: x/bank/keeper/msg_server.go 
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See full example on GitHub msgServer methods can retrieve the sdk.Context from the context.Context parameter method using the sdk.UnwrapSDKContext: x/bank/keeper/msg_server.go 
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See full example on GitHub sdk.Msg processing usually follows these 3 steps:

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Validation​

The message server must perform all validation required (both stateful and stateless) to make sure the message is valid. The signer is charged for the gas cost of this validation. For example, a msgServer method for a transfer message should check that the sending account has enough funds to actually perform the transfer. It is recommended to implement all validation checks in a separate function that passes state values as arguments. This implementation simplifies testing. As expected, expensive validation functions charge additional gas. Example: 
ValidateMsgA(msg MsgA, now Time, gm GasMeter) error {    if now.Before(msg.Expire) {        return sdkerrrors.ErrInvalidRequest.Wrap("msg expired")    }    gm.ConsumeGas(1000, "signature verification")    return signatureVerificaton(msg.Prover, msg.Data)}
Previously, the ValidateBasic method was used to perform simple and stateless validation checks. This way of validating is deprecated, this means the msgServer must perform all validation checks.

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State Transition​

After the validation is successful, the msgServer method uses the keeper functions to access the state and perform a state transition.

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Events​

Before returning, msgServer methods generally emit one or more events by using the EventManager held in the ctx. Use the new EmitTypedEvent function that uses protobuf-based event types: 
ctx.EventManager().EmitTypedEvent(    &group.EventABC{Key1: Value1,  Key2, Value2})
or the older EmitEvent function: 
ctx.EventManager().EmitEvent(    sdk.NewEvent(        eventType,  // e.g. sdk.EventTypeMessage for a message, types.CustomEventType for a custom event defined in the module        sdk.NewAttribute(key1, value1),        sdk.NewAttribute(key2, value2),    ),)
These events are relayed back to the underlying consensus engine and can be used by service providers to implement services around the application. Click here to learn more about events. The invoked msgServer method returns a proto.Message response and an error. These return values are then wrapped into an *sdk.Result or an error using sdk.WrapServiceResult(ctx sdk.Context, res proto.Message, err error): baseapp/msg_service_router.go 
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See full example on GitHub This method takes care of marshaling the res parameter to protobuf and attaching any events on the ctx.EventManager() to the sdk.Result. proto/cosmos/base/abci/v1beta1/abci.proto 
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See full example on GitHub This diagram shows a typical structure of a Protobuf Msg service, and how the message propagates through the module. Transaction flow

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Telemetry​

New telemetry metrics can be created from msgServer methods when handling messages. This is an example from the x/auth/vesting module: x/auth/vesting/msg_server.go 
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See full example on GitHub